Lubricating coating composition suitable for tubular threaded joints

ABSTRACT

A tubular threaded joint for connecting tubing or casing used to excavate oil wells have a lubricating coating formed from a lubricating coating composition comprising one or more basic lubricants selected from a basic sulfonate salt, a basic salicylate salt, a basic phenate salt, and a basic carboxylate salt and having biodegradability value (BOD) of at least 20% when measured after 28 days in seawater. The joint can be used without application of a compound grease containing heavy metals even in a country or region having severe environmental regulations.

TECHNICAL FIELD

This invention relates to a lubricating coating composition suitable for lubricating coating treatment of tubular threaded joints which are used to connect oil country tubular goods (abbreviated as OCTG) to each other.

A tubular threaded joint having a lubricating coating formed by treatment with a composition according to the present invention can be used to connect OCTG without application of a lubricating grease which contains a large amount of heavy metals and hence raises the a concern of causing environmental pollution, even if the joint is of the type having an unthreaded metal-to-metal contact portion which provides the joint with improved sealability but which makes the joint susceptible to galling.

BACKGROUND ART

OCTG are tubing and casing which are used to excavate oil wells. They are normally assembled on site by connecting steel tubes having a length on the order of ten some meters to each other using tubular threaded joints. Conventionally, the depth of oil wells has been 2,000-3,000 meters, but in recent deep sea oil fields, it may reach 8,000-10,000 meters.

In its environment of use, a threaded joint for connecting OCTG is subjected not only to a load in the form of an axial tensile force caused by the weight of OCTG and joints connected thereto but also to a combined internal and external pressure and subterranean heat. Therefore, it must maintain an airtight connection without breakage even in such a severe environment.

During the operation of lowering tubing or casing into an oil well, a tubular threaded joint which has once been tightened is sometimes loosened and then retightened. API (the American Petroleum Institute) requires a tubular threaded joint for OCTG to have sufficient galling resistance to make it possible to carry out tightening (makeup) and loosening (breakout) ten times for a joint for tubing and three times for a joint for casing without the occurrence of galling (unrepairable severe seizure) while maintaining airtightness.

A tubular threaded joint having good sealability when used to connect OCTG is of the pin-box structure capable of forming a metal-to-metal contact seal. With a typical tubular threaded joint of this type, a pin is formed on the outer surface of each end portion of a steel tube and includes a threaded portion with a male (external) thread and an unthreaded metal-to-metal contact portion, while a box is formed on the inner surface of a coupling, which is a separate connecting member, and includes a threaded portion with a female (internal) thread and an unthreaded metal-to-metal contact portion. The tubular threaded joint is tightened by inserting the pin into the box and tightening the male and female threads until the unthreaded metal-to-metal contact portions of the pin and the box tightly contact each other to form a metal-to-metal contact seal. Prior to tightening of the joint, a lubricating grease called compound grease is usually applied to the surfaces of the threaded portions and the unthreaded metal-to-metal contact portions which are the engaging portions of the joint when it is tightened, in order to provide these portions with improved galling resistance and airtightness. The engaging portions of a tubular threaded joint may be pretreated so as to have an increased surface roughness by surface treatment such as phosphating in order to increase the retention of a compound grease.

However, a compound grease contains a large amount of powder of heavy metals such as lead, zinc, and copper in order to provide the compound grease with sufficient lubricity and corrosion resistance. Therefore, the applied grease causes environmental pollution if it is washed off or squeezed out to its surroundings. In addition, the process of applying a compound grease worsens the work environment and decreases the efficiency of the operation of assembling OCTG. Accordingly, there has been a demand for a tubular threaded joint which fulfills its function sufficiently without application of a compound grease.

With the aim of providing a tubular threaded joint which can be used without application of a compound grease, a lubricating coating composition for forming a lubricating coating on the engaging portions of a tubular threaded joint and a lubricating coating or layer formed from such a composition are proposed in the following Japanese patent documents:

Patent Document 1: JP 2002-173692 A1

Patent Document 2: JP 2004-53013 A1

Patent Document 3: JP 2004-507698 A1.

DISCLOSURE OF THE INVENTION

Since the OSPAR (Oslo-Paris) Convention pertaining to preventing maritime pollution in the Northeast Atlantic came into effect in 1998, strict environmental regulations have been increasing on a global scale. In the excavation of gas wells and oil wells on offshore rigs, in order to minimize the discharge of causative agents of marine pollution, there is a recent trend that any substance which is used on a rig and which is capable of being discharged to the environment requires an environmental impact assessment thereof. If the substance does not meet the standards in that country or region, use of the substance is prohibited.

The properties to be assessed in this environmental impact assessment are prescribed in the OSPAR Convention as the HOCNF (Harmonized Offshore Chemical Notification Format). Among these properties, biodegradability (abbreviated as BOD) is particularly important.

For a tubular threaded joint, an environmental impact assessment of a compound grease is of course required, since its use unavoidably involves application and washing operations on a rig. Lubricating coatings and coating compositions for forming such a coating as proposed in the above-listed patent documents should also be subjected to such an assessment since there is a possibility of such a coating being discharged to its surroundings during washing of a joint.

However, although the lubricity and anticorrosive properties of lubricating coatings and coating compositions which have been proposed in the prior art are taken into consideration, their compositions are not designed with consideration of biodegradability, which is now an important item requiring assessment, and they have not been assessed with respect to their environmental impact. Therefore, their use has become difficult under recent conditions in which environmental regulations are becoming increasingly strict.

It is an object of the present invention to provide a tubular threaded joint suitable for use in connecting oil country tubular goods (OCTG) which eliminates the above-described problems of the prior art.

A more specific object of the present invention is to provide a tubular threaded joint which can be used to connect OCTG without application of a lubricating grease such as a compound grease and without problems related to lubricity, which is prevented from rusting and exhibits improved galling resistance and airtightness, and which can be used even in countries or regions having strict environmental regulations.

Another object of the present invention is to provide a lubricating coating composition for use in the manufacture of such a tubular threaded joint.

In order to achieve the above objects, the present inventors carried out investigations on various lubricating coating compositions suitable for a tubular threaded joint with respect to biodegradability, lubricity, and anticorrosive properties.

(A) Biodegradability (BOD)

In order to determine the assessment of environmental impact on the ocean, the following testing methods are generally employed to assess the biodegradability of a substance in seawater:

(a) OECD guidelines for testing of chemicals—1992 OECD 306: Biodegradability in Seawater, Closed Bottle Method; and

(b) Modified seawater variant of ISO TC/147, SC5/WG4 N141 1990: BOD test for insoluble substances.

Among these methods, the one which is more suitable for the particular sample should be employed. In either testing method, the test result is indicated in percent (e.g., BOD=15%), and the larger the BOD value, the higher the biodegradability, which indicates a lower impact on the environment.

The biodegradability assessed by either method is acceptable if the BOD value measured after 28 days (hereinafter referred to as BOD₂₈) is at least 20% or “BOD₂₈≧20%”. At present, the minimum acceptable BOD value differs between countries or regions, but the criterion “BOD₂₈≧20%” can meet the minimum acceptable level of BOD for substances which can be used on an offshore rig according to the regulations in Norway, which are said to be the strictest.

A biodegradability test for use in designing the composition of a lubricating coating composition can be carried out separately on each of the candidate components in the composition. However, in view of the situation of shipment of tubular threaded joints as a product, the final judgement of biodegradability should be made based on an overall assessment of a lubricating coating composition which combines the assessments of the individual components.

(B) Lubricity

A lubricant based on any of a basic sulfonate salt, a basic salicylate salt, a basic phenate salt, and a basic carboxylate salt is in the form of a grease-like semisolid at room temperature and exhibits fluidity under hydrostatic pressure. When their lubricity was evaluated by the number of tightening and loosening cycles before galling occurred in a repeated tightening and loosening test using actual tubular threaded joints, it was found that these basic lubricants exhibit good galling resistance even with a relatively thin coating.

(C) Anticorrosive Properties

When the anticorrosive properties of sulfonate salts, salicylate salts, phenate salts, and carboxylate salts were evaluated by a salt spray test according to JIS Z2371, it was found that a basic salt had better anticorrosive properties than a neutral salt for each of these salts.

Upon further investigation based on the finding that any of a basic sulfonate salt, a basic salicylate salt, a basic phenate salt, and a basic carboxylate salt can exhibit good lubricity and good anticorrosive properties with a relatively thin coating, it was found that this type of a basic lubricant does not have good biodegradability but can still satisfy the target criterion “BOD₂₈≧20%” by the addition of one or more other lubricant components thereto to form a lubricating coating composition while maintaining good lubricity and anticorrosive properties.

The present invention provides a lubricating coating composition comprising at least one basic lubricant selected from a basic sulfonate salt, a basic salicylate salt, a basic phenate salt, and a basic carboxylate salt wherein the composition has a biodegradability value (BOD) of at least 20% when measured after 28 days in seawater.

In a preferred embodiment, a lubricating coating composition according to the present invention further comprises at least one additional lubricant selected from those having a higher (greater) biodegradability than that of the basic lubricant. The additional lubricant is preferably selected from a fatty acid metal salt and a wax, and more preferably it comprises at least one fatty acid metal salt and at least one wax. The fatty acid metal salt is preferably selected from alkaline earth metal salts of stearic acid or oleic acid.

A preferred chemical composition of a lubricating coating composition according to the present invention may contain up to 30 mass % of a volatile organic dissolving medium (solvent), and the remainder comprises, when the total amount of the remainder composition is taken as 100 parts by mass, 55-75 parts by mass of a basic lubricant, 20-25 parts by mass of a fatty acid metal salt, and 10-20 parts by mass of a wax.

The present invention also provides a tubular threaded joint constituted by a pin and a box each having a threaded portion and an unthreaded metal-to-metal contact portion as engaging portions, characterized in that the surfaces of the engaging portions of at least one of the pin and the box are coated with the above-described lubricating coating composition to a thickness of at least 10 micrometers, thereby allowing the joint to be tightened without application of a compound grease.

In the present invention, the term “lubricant” indicates a lubricity improving agent. A pin is a member of a tubular threaded joint which has a male (external) threaded portion, while a box is the other member of the joint having a female (internal) threaded portion.

A tubular threaded joint having a lubricating coating formed from a lubricating coating composition according to the present invention has high biodegradability and still exhibits satisfactory galling resistance and anticorrosive properties on the same level as obtained with application of a compound grease. As a result, it can be used without application of a compound grease and without any concern for environmental pollution even in a country or region having strict environmental regulations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a steel pipe for OCTG and a coupling which are assembled together for shipment.

FIG. 2 is a schematic diagram showing a tubular threaded joint having a threaded portion and an unthreaded metal-to-metal contact portion.

FIG. 3 is a schematic diagram showing minute gaps in a threaded portion and an unthreaded metal-to-metal contact portion of a tubular threaded joint.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following description, any percent relating to a chemical composition is by mass unless otherwise indicated.

The components which can constitute a lubricating coating composition according to the present invention are described below individually.

[Dissolving Medium]

A dissolving medium or solvent can be used in order to dissolve or disperse the basic lubricant and one or more optional lubricants as well as other additives, if used, thereby facilitating the formation of a lubricating coating having a uniform thickness and a uniform composition in an efficient manner. Therefore, if a satisfactory lubricating coating can be formed with the lubricant components alone, it is unnecessary to use a dissolving medium in a coating composition.

In the present invention, since the principal component of a lubricating coating is one or more lubricants, an organic dissolving medium is used. There is no limitation on the type of dissolving medium as long as it has a high dissolving power for the lubricant components in the composition and easily volatilizes. Preferred dissolving media are petroleum solvents such as those corresponding to industrial gasoline prescribed in JIS K2201 and including solvent and mineral spirit, aromatic petroleum naphtha, xylene, and Cellosolves. A dissolving medium having a flash point of 30° C. or higher, an initial boiling temperature of 150° C. or higher, and a final boiling point of 210° C. or lower is suitable due to its ease of handling, rapid evaporation, and short drying time. From the standpoint of biodegradability, mineral spirit is preferable.

The biodegradability of an organic dissolving medium is generally not so high. Therefore, if it is present in a lubricating coating composition in a large amount, the biodegradability of the entire composition is worsened. The amount of a dissolving medium is preferably selected, as long as the biodegradability of a lubricating coating composition satisfies the conditions for BOD defined by the present invention, such that the dissolving medium can improve the wettability of the is surface to be coated and the spreadability of the coating composition and facilitate adsorption of the below-described lubricity improving agents (lubricants) by the surface, in addition to its intended function of dissolving or uniformly dispersing the lubricants.

The amount of a dissolving medium in a lubricating coating composition is preferably in the range of from 0% to 30% and more preferably from 5% to 25%. If the amount is too small, the viscosity of the lubricating coating composition may be so high that it is difficult for the composition to form a uniform coating and exhibit the above-described adsorbing function. If it is too large, it is difficult for the composition to have a desired biodegradability.

[Basic Lubricant]

In a composition according to the present invention, at least one basic lubricant selected from basic sulfonate salts, basic salicylate salts, basic phenate salts, and basic carboxylate salts is used as a principal component of lubricity improving agents (lubricants). The term “principal component” does not always mean that it is present in the largest amount, but indicates that the basic lubricant performs a main role to achieve the desired lubricating performance.

All the above-described four classes of basic lubricants are a salt formed from an aromatic acid and an excess alkali. At room temperature, they are a grease-like semisolid substance comprising an oil and the excess alkali dispersed in the oil in the form of colloidal microparticles.

The biodegradability of any of these basic lubricants is low and cannot satisfy the criterion “BOD₂₈≧20%” by itself. Thus, as the amount of a basic lubricant in a lubricating coating composition increases, the biodegradability thereof tends to decrease. Accordingly, a basic lubricant is present in a lubricating coating composition in an amount which is effective at improving the galling resistance and anticorrosive properties of the composition, provided that the composition as a whole has a biodegradability satisfying the above criterion. The amount of a basic lubricant is preferably in the range of 55 to 70 parts by mass when the total mass of the composition excluding the above-described dissolving medium (namely, the total amount of all the nonvolatile components which constitute a lubricating coating) is taken as 100 parts by mass.

Among the four classes of the above-mentioned basic lubricants, a basic sulfonate salt is most advantageous in terms of lubricity and anticorrosive properties. The sulfonic acid constituting the sulfonate salt may be either petroleum sulfonic acid obtained by sulfonating aromatic components of petroleum fractions or a synthetic sulfonic acid. Examples of the synthetic sulfonic acid include dedecylbenzenesulfonic acid, dinonylnaphthalenesulfonic acid, and the like. The salt of a sulfonic acid may be either an alkali metal salt or an alkaline earth metal salt. Preferably it is an alkaline earth metal salt and more preferably a calcium salt (namely, a basic calcium sulfonate). Similarly, with respect to a basic salicylate salt, a basic phenate salt, and a basic carboxylate salt, an alkaline earth metal salt and particularly a calcium salt is preferred.

Below, the present invention will be described with respect to an embodiment in which the basic lubricant is a basic calcium sulfonate, but the invention is not restricted to this embodiment. The following explanation is generally applicable to other embodiments in which the basic lubricant is a basic sulfonate other than a basic calcium sulfonate or it is a basic salicylate salt, a basic phenate salt, or a basic carboxylate salt, although the amount of the basic lubricant can be adjusted taking into consideration the biodegradability and other properties of the composition.

A basic calcium sulfonate which can be used in the present invention is a known substance and is commercially available under the trade name Sulfol 1040 from Matsumura Oil Research Corp. and under the tradename Lubrizol 5318 from Lubrizol Corp, for example.

A basic calcium sulfonate may be prepared by dissolving a neutral sulfonate salt in a solvent, which can be suitably selected from aromatic hydrocarbons, alcohols, and mineral oil, and then adding to the resulting solution an amount of calcium hydroxide required to form a desired basic calcium sulfonate followed by mixing. Carbon dioxide gas is subsequently passed through the mixture in an excess amount so as to sufficiently carbonate the added calcium hydroxide, and the reaction mixture is filtered after addition of a filter aid such as activated clay. The desired basic calcium sulfonate is obtained by distilling the filtrate at a reduced pressure to is remove the volatile solvent.

In either form of a commercially available product and a synthetic product, a basic calcium sulfonate is a grease-like semisolid substance which contains calcium carbonate in the form of colloidal microparticles which are stably dispersed in an oil. The dispersed microparticles of calcium carbonate function as a solid lubricant and enable the basic sulfonate salt to exhibit significantly improved lubricity over common liquid lubricating oil particularly under severe tightening conditions having a large amount of thread interference. When this lubricant works between frictional surfaces having minute irregularities (surface roughness), it can exhibit an even more improved galling-preventing effect due to the micro lubricating effect by hydrostatic fluid pressure of the oil combined with the solid lubrication action of the microparticles. This effect can be similarly achieved with other basic lubricants.

As the base number (as specified in JIS K2501) of the basic lubricant which is used increases, its lubricity (galling resistance) tends to increase, since the amount of calcium carbonate microparticles serving as a solid lubricant increases. In addition, if the lubricant has a basicity higher than a certain level, it can exert its activity of neutralizing an acidic substance effectively, thereby making it possible to provide a lubricating coating with an increased anti-rust ability. For these reasons, it is preferred that the basic lubricant which is used in the present invention have a base number (according to JIS K2501) of at least 50 mg-KOH/g. If two or more basic lubricants are used, the base number is a weighted average of their base numbers. However, if the lubricant has a base number exceeding 500 mg-KOH/g, its hydrophilic nature is increased, leading to a decrease in anticorrosive properties and easy occurrence of rusting. A preferred range for the base number of the basic lubricant is from 100 to 500 mg-KOH/g and a more preferred range is from 250 to 450 mg-KOH/g.

[Additional Lubricants]

In addition to the above-described basic lubricant which is a principal lubricant, a lubricating coating composition according to the present invention preferably contains one or more additional lubricants as lubricity improving agents. Those having better (higher) biodegradability than the basic lubricant are used as the is additional lubricants. As a result, even in the case where a highly basic lubricant which does not have such good biodegradability is used as a principal lubricant, it is possible to obtain a lubricating coating composition satisfying the condition of biodegradability according to the present invention.

Additional lubricants are preferably selected from fatty acid metal salts and waxes. More preferably, at least one fatty acid metal salt and at least one wax are used as additional lubricants.

<Fatty Acid Metal Salt>

An alkaline earth metal salt of a fatty acid is preferably used as a fatty acid metal salt, since fatty acid salts with other metals have inferior biodegradability or are not preferred from an environmental standpoint. A fatty acid metal salt is active as a lubricant, although its activity is lower than that of the above-described basic lubricant such as a basic calcium sulfonate.

The fatty acid portion of the salt is preferably one having 12 to 30 carbon atoms from the viewpoints of lubricity and anticorrosive properties. The fatty acid may be either a mixed fatty acid derived from natural fat or fatty oil such as beef tallow, lard, wool fat, palm oil, rape-seed oil, and coconut oil, or a single fatty acid such as lauric acid, tridecanoic acid, myristic acid, palmitic acid, lanopalmitic acid, stearic acid, isostearic acid, oleic acid, elaidic acid, arachic acid, behenic acid, erucic acid, lignoceric acid, and lanoceric acid.

Particularly preferable fatty acid metal salts in terms of biodegradability are alkaline earth metal salts of stearic acid or oleic acid, and a calcium salt of such an acid is particularly suitable. The fatty acid metal salt may be either a neutral salt or a basic salt.

There is no limit on the amount of a fatty acid metal salt in a lubricating coating composition, and it may be 0%. However, such a salt is usually added in at least a certain amount in order to provide the composition with the desired biodegradability. A preferred amount of the fatty acid metal salt is in the range of from 20 to 25 parts by mass when the total mass of the composition excluding the dissolving medium is taken as 100 parts by mass. If this amount is too large, the amount of the basic lubricant which is a principal lubricant becomes relatively small, leading to a decrease in lubricity.

<Wax>

A wax may be added in order to enhance the biodegradability of a lubricating coating composition, although its lubricity is lower than that of the above-described basic lubricant.

Examples of waxes which can be used include animal waxes such as beeswax and whale tallow; vegetable waxes such as Japan wax, carnauba wax, candelilla wax, and rice wax; mineral waxes such as paraffin wax, microcrystalline wax, petrolatum, montan wax, ozokerite, and ceresin; and synthetic waxes such as oxide wax, polyethylene wax, Fischer-Tropsch wax, amide wax, and hardened castor oil (castor wax). Of these, petrolatum, which is a kind of mineral wax, is preferred from the standpoint of biodegradability.

There is no limit on the amount of a wax in a lubricating coating composition, and it may be 0%. However, at least certain amount of wax is usually added in order to provide the composition with the desired biodegradability. A preferred amount of the wax is in the range from 10 to 20 parts by mass when the total mass of the composition excluding the dissolving medium is taken as 100 parts by mass. If this amount is too large, the amount of the basic lubricant which is a principal lubricant becomes relatively small, leading to a decrease in lubricity.

[Tubular Threaded Joint]

A tubular threaded joint is constituted by a pin and a box each having a threaded portion and an unthreaded metal-to-metal contact portion as engaging portions. A lubricating coating composition according to the present invention can be applied to the surface of the engaging portions of at least one of the pin and the box.

Typically, a tubular threaded joint is shipped in the state shown in FIG. 1. Namely, a steel pipe A for oil country tubular goods (OCTG) has a pin with a male threaded portion 3 formed on the outer surface at both ends thereof, and a coupling B has a box with a female threaded portion 4 formed on the inner surface at both sides thereof. Before shipping, the coupling B is connected to one end of the steel pipe A. In this figure, for simplicity, an unthreaded metal-to-metal contact portion is omitted.

A tubular threaded joint is not limited to the type illustrated in FIG. 1. A different type of tubular threaded joint called an integral type can also be used. This type, which has a pin with a male thread on the outer surface at one end of a steel pipe for OCTG and a box with a female thread on the inner surface at the other end of the pipe, does not need to use a coupling for tightening. In addition, it is also possible to form a box at both ends of a steel pipe for OCTG and a pin on both sides of a coupling. Below, the present invention will be described with respect to an example of a tubular threaded joint having the form shown in FIG. 1.

FIG. 2 schematically shows a cross section of the connecting portion of a tubular threaded joint. In this figure, 1 is a pin, 2 is a box, 3 is a male (external) thread portion, 4 is a female (internal) thread portion, and 5 is an unthreaded metal-to-metal contact portion of each of the pin and box. The male and female threaded portions 3 and 4 and the unthreaded metal-to-metal contact portions 5 of the pin and box serve as engaging portions of the tubular threaded joint and form frictional interfaces during tightening of the joint. According to the present invention, a lubricating coating composition is applied to the engaging portions of at least one of the pin and box (i.e., to the threaded portion 3 or 4 and the unthreaded metal-to-metal contact portion 5) so as to form a grease-like semisolid lubricating coating.

The pin and the box are shaped so as to interfit with each other, but when they are observed in detail, as shown in FIG. 3, there are minute gaps 6 particularly between mating male and female threads. In the absence of these gaps between mating threads, tightening of a tubular threaded joint becomes practically impossible. In addition, a larger minute gap 6 usually exists between the unthreaded metal-to-metal contact portions and the thread portions of the pin and box as depicted. A lubricating composition such as a compound grease is retained in these gaps and can seep out to their surroundings under the pressure exerted during tightening and thereby prevent galling, so these minute gaps contribute to lubrication. A lubricating coating formed from a lubricating coating composition according to the present invention is semisolid like a compound grease and thus can seep out, thereby providing improved lubricity and airtightness to the joint.

Also like a compound grease, a lubricating coating composition according to the present invention has improved anticorrosive properties. As a result, after a tubular threaded joint has been shipped in the state shown in FIG. 1, the engaging portions of the joint to which the lubricating coating composition is applied are protected from rusting.

[Thickness of Lubricating Coating]

A primary purpose of the formation of a lubricating coating in a tubular threaded joint is prevention of galling even under severe lubrication conditions which may be accompanied by plastic deformation as encountered when the pressure applied to the joint is locally excessive due to misalignment or inclination of the joint caused by some problems in assembling the joint for tightening or due to incorporation of foreign matter. For this purpose, it is essential to introduce lubricants (lubricity improving agents) into the frictional interfaces and maintain the lubricants therein.

Accordingly, a lubricating coating composition must be applied in an amount sufficient to fill the minute gaps 6 such as those between mating threads in the engaging portions of a tubular threaded joint. If the applied amount is too small, it becomes impossible to expect the lubricants to seep into frictional interfaces or seep into a gap from other gaps under the action of the hydrostatic pressure generated by tightening. For this purpose, the thickness of a lubricating coating is preferably at least 10 micrometers.

Since the engaging portions of a box and a pin contact each other when a tubular threaded joint is tightened, in the interest of achieving lubricity, it is sufficient to form a lubricating coating on the engaging portions of only one of the pin and box. However, in order to also provide the engaging portions with anticorrosive properties, it is necessary to form a lubricating coating on the engaging portions of both the pin and box. The minimum thickness necessary for anticorrosive properties is also 10 micrometers. Therefore, a lubricating coating with a thickness of at least 10 micrometers is preferably formed on the engaging portions of both members. However, when a coupling is connected to one end of a steel pipe for OCTG as shown in FIG. 1 before shipping, the pin and the box on the connected side are protected from rusting by forming a lubricating coating on the engaging portions of only one of the pin and the box. Even in such a case, on the opposite non-connected sides of the pin and the box (the left-hand pin and the right-hand box), it is preferable that both the pin and box have a lubricating coating.

A lubricating coating formed from a lubricating coating composition according to the present invention does not need to be made extremely thick since it contains a basic lubricant such as a basic calcium sulfonate which has a significantly high lubricity. Too thick a coating not only wastes materials, but works against the goal of preventing environmental pollution, which is an important object of the present invention. Although the upper limit of the coating thickness is not limited, it is preferably approximately 200 micrometers.

A more preferable thickness of the lubricating coating is in the range of 30 to 150 micrometers. However, when the surface roughness is increased as described below, the thickness of the lubricating coating is preferably larger than the value of the surface roughness (Rmax). The thickness of a lubricating coating formed on a rough surface is defined in the present invention as the mean value of the smallest thickness and the largest thickness.

When a lubricating coating composition according to the present invention contains a dissolving medium, the composition itself can be in the form of a liquid having good applicability rather than a semisolid at room temperature, and it can be applied as is without heating. Once such a composition is applied to the engaging portions of a tubular threaded joint, the dissolving medium, which is generally volatile, vaporizes from the applied coating and leaves a semisolid lubricating coating. Application of the lubricating coating composition can be carried out by any suitable coating method such as brush coating, dip coating, or spray coating.

When the viscosity of a lubricating coating composition is too low for it to be applied at room temperature as in the case where the amount of a dissolving medium is small or even zero, it can be applied after heating to a temperature sufficient to lower the viscosity to such a degree that the composition can be applied easily.

[Surface Roughness of Engaging Portions]

With a tubular threaded joint treated with a lubricating coating composition according to the present invention to form a lubricating coating on the surfaces of its engaging portions such as a threaded portion and an unthreaded metal-to-metal contact portion, if the surface roughness of the engaging portions, which is 3-5 micrometers in an as-machined state, is increased by a suitable means, its galling resistance is further improved. This is because the functions of lubricants of seeping into frictional interfaces or seeping into a gap from other gaps under the action of the hydrostatic pressure generated in the engaging portion by tightening are caused by lubricants confined in minute indentations of the surface roughness. The intensity of these functions depends on the magnitude of the surface roughness regardless of the method of forming the surface roughness. A range of surface roughness suitable for improvement in galling resistance is 5-40 micrometers expressed as Rmax. If the surface roughness exceeds 40 micrometers Rmax, sufficient sealing cannot be obtained in the peripheries of indentations, and the desired hydrostatic pressure is not generated so that sufficient lubrication is not obtained. A more preferable range of Rmax is from 10 to 30 micrometers.

<Method of Surface Roughening>

Although there is no restriction on the method of surface roughening, the following methods are possible:

(1) Blasting with sand or grid as abrasive grains: The surface roughness which is obtained can be controlled by the size of the abrasive grains.

(2) Etching with an acid: The surface is roughened by immersion in a strongly acidic solution such as sulfuric acid, hydrochloric acid, nitric acid, or hydrofluoric acid.

(3) Phosphating: A chemical conversion coating with a phosphate such as manganese phosphate, zinc phosphate, iron-manganese phosphate, or zinc-calcium phosphate is formed. As the phosphate crystals deposited on the surface by phosphating grow, the roughness of the crystal surface increases.

(4) Electroplating: Copper plating or iron plating is suitable. Electroplating occurs preferentially in protruded portions on the surface, leading to a slight increase in surface roughness.

(5) Dry-process impact plating: This is a plating method such as zinc blasting or zinc-iron alloy blasting in which particles having an iron core coated with a metallic material for plating (such as a zinc or a zinc alloy) are blasted onto a surface to be plated using centrifugal force or air pressure.

It is easier to apply these treatment methods for surface roughening to the surface of a box, but they can be applied to the surface of a pin or the surfaces of both the pin and box. Methods (3) through (5) result in the formation of an undercoat layer having an increased surface roughness, which prevents direct metal-to-metal contact in the engaging portions after a lubricant coating has been lost, so these methods are preferred in that galling resistance and anticorrosive properties are simultaneously improved. In particular, a coating of manganese phosphate is preferable since it is made of bristling acicular crystals and hence can easily achieve a large surface roughness, which is good at retaining a large amount of lubricants.

Some materials of a steel pipe for OCTG such as high alloy steels cannot undergo phosphating directly. In such cases, phosphating can be performed after the iron plating described in (4) above is initially applied. When an undercoat layer is formed as in methods (3) to (5) above, the thickness of the undercoat layer is preferably greater than the surface roughness of the undercoat layer, since the layer has good retention of lubricants and good adhesion. Thus, the thickness of the undercoat layer is preferably in the range of 5-40 micrometers.

EXAMPLES

The following examples are presented to further illustrate the present invention. These examples are to be considered in all respects as illustrative and not restrictive.

In the examples, lubricating coating compositions were prepared and their biodegradability was determined using the following two methods (a) and (b), which are commonly employed for evaluation of biodegradability of a substance in seawater in environmental impact assessment:

(a) OECD guidelines for testing of chemicals—1992 OECD 306: Biodegradability in Seawater, Closed Bottle Method; and

(b) Modified seawater variant of ISO TC/147, SC5/WG4 N141 1990: BOD test for insoluble substances.

Specifically, the biodegradability of each component in a coating composition after 28 days (BOD₂₈) in seawater was determined by one of the above methods selected suitably therefor. More specifically, method (a) was used for mineral spirit, petrolatum wax, and polyethylene resin powder (used in a comparative example), while method (b) was used for basic calcium sulfonate and calcium stearate.

As the final judgement of biodegradability, the overall biodegradability of the lubricating coating composition as a whole was determined by combining the results of the individual components. The value of the overall biodegradability was calculated as a weighted average of the values of biodegradability of the individual components by considering their contents in the composition.

The lubricity of each lubricating coating composition was tested using a tubular threaded joint constituted by a pin formed on the outer surface at both ends of a steel pipe for OCTG (outer diameter of 17.8 cm=7 inches) made from either carbon steel or 13Cr steel having the compositions shown in Table 1 and a box formed on the inner surface of a coupling of the same steel material.

All the tubular threaded joints used for testing were of the type having a threaded portion and an unthreaded metal-to-metal contact portion on each of the pin and box and capable of forming a metal-to-metal seal. In the following description, the engaging surfaces of a pin, which include the surface of its threaded portion and unthreaded metal-to-metal contact portion, will be referred to as the “pin surface”, and the engaging surfaces of a box which includes the surface of its threaded portion and unthreaded metal-to-metal contact portion will be referred to as the “box surface”.

The tubular threaded joints underwent the following surface treatment for surface roughening. The pin surface of a tubular threaded joint of the carbon steel was treated by zinc phosphating, and the box surface thereof was treated by manganese phosphating. The box surface of a tubular threaded joint of the 13Cr steel was coated with copper plating, and the pin surface thereof remained as machined without surface treatment. The 13Cr steel is a kind of high alloy steel and is more susceptible to galling than the carbon steel.

A lubricating coating composition to be tested was applied by brush coating only to the box surface of a tubular threaded joint which had been surface treated as described above so as to form a lubricating coating having a thickness of 30 micrometers.

The tubular threaded joint having a lubricating coating formed from the composition on the box surface was subjected to a repeated tightening and loosening test with a tightening torque of 20,000 N-m for up to ten cycles, and the lubricity of the lubricating coating composition was evaluated by the number of tightening cycles before galling occurred in the test.

The anticorrosive properties of each lubricating coating composition were evaluated by a salt spray test (SST) specified in JIS Z2371 using a test sheet with dimensions of 50 mm×100 mm and 2 mm in thickness made of the carbon steel or the 13Cr steel having the compositions shown in Table 1. A lubricating coating to be tested was formed to a thickness of 30 micrometers from each composition on the test sheet as machined without surface treatment. The coated test sheet was subjected to the salt spray test for 1000 hours, and the presence or absence of rust was determined visually.

The compositions of the lubricating coating compositions which were tested and the test results are shown in Table 2.

TABLE 1 C Si Mn P S Cu Ni Cr Mo Carbon 0.24 0.3 1.3 0.02 0.01 0.04 0.07 0.17 0.04 steel 13Cr steel 0.19 0.25 0.8 0.02 0.01 0.04 0.1 13 0.14 (Contents in mass %, the balance being Fe and incidental impurities)

TABLE 2 Comparative Conventional Example 1 Example 2 Example Example Lubricating Mineral spirit 25% 15% Grease coating Basic Ca sulfonate 50% (66.7)* 65% (65)* 72% (84.7)* specified by composition Calcium stearate 17% (22.7)* 23% (23)* API (mass %) Petrolatum wax  8% (10.6)* 12% (12)* 8% (9.4)* Polyethylene powder 5% (5.9)* Ease of application(*1) Good Fair Good Good Biodegrad- BOD₂₈(*2) 26% 36% 12% Poor(*4) ability Assessment(*3) Good Good Poor (in seawater) Lubricity(*5) Carbon steel 10 cycles 10 cycles 10 cycles 10 cycles 13Cr steel 10 cycles 10 cycles 10 cycles 10 cycles Anticorrosive Carbon steel No rust No rust No rust No rust properties in 13Cr steel No rust No rust No rust No rust SST *The numerals in parentheses are parts by mass based on 100 parts by mass of the total amount of the composition excluding the dissolving medium (mineral spirit). (*1)Ease of application: Good: applicable at room temperature; Fair: applicable only after lowering the viscosity of the composition by heating. (*2)Overall assessment of the results of the individual components measured by either of the following methods: “OECD guidelines for testing of chemicals - 1992 OECD 306: Biodegradability in Seawater, Closed Bottle Method” and “modified seawater variant of ISO TC/147, SC5/WG4 N141 1990: BOD test for insoluble substances”. (*3)Assessment of biodegradability: Good: BOD₂₈ ≧ 20% (reached the target value for the present invention) Poor: BOD < 20% (did not reach the target value for the present invention) (*4)Cannot be used irrespective of its BOD value due to the presence of heavy metals such as lead contained therein. (*5)Number of tightening cycles before galling occurred in a repeated tightening and loosening test up to ten cycles.

Example 1

A lubricating coating composition was prepared which contained 25% of mineral spirit as a dissolving medium, 50% (66.7 parts) of basic calcium sulfonate having a base number of 400 mg-KOH/g as a basic lubricant, and 17% (22.7 parts) of calcium stearate and 8% (10.6 parts) of petrolatum wax both as additional lubricants. The parts in parentheses are the amounts of the respective components in parts by mass based on 100 parts by mass of the total amount of the components in the composition excluding the dissolving medium.

This lubricating coating composition contained a dissolving medium and thus had a low viscosity and a high spreadability, so it was easy to apply and could be applied by brush coating to the box surface of a test threaded joint to be used in a repeated tightening and loosening test and the surface of a test sheet for a salt spray test while remaining at room temperature. On the other hand, due to the presence of a dissolving medium which does not have good biodegradability, the biodegradability (BOD₂₈) of the entire lubricating coating composition in seawater was 26%, which was higher than the minimum acceptable value of 20% but was lower than that of the composition of Example 2 containing no dissolving medium.

In the repeated tightening and loosening test using a test threaded joint having a lubricating coating formed from the composition, 10 cycles of tightening and loosening which are required for a joint for tubing could be performed without the occurrence of galling both with a joint made of the carbon steel and one made of 13Cr steel. In addition, in the salt spray test for 1000 hours using a test sheet having a lubricating coating formed from the composition, no rust was found on either the carbon steel or the 13Cr steel.

Example 2

A lubricating coating composition was prepared which did not contain a dissolving medium but contained 65% of the same basic calcium sulfonate as used in Example 1 as a basic lubricant, and 23% of calcium stearate and 12% of petrolatum wax both as additional lubricants.

Since this lubricating coating composition did not contain a dissolving medium and thus had a high viscosity at room temperature, it was previously heated to 60° C. to lower its viscosity and then applied by brush coating to the box surface of a test threaded joint to be used in a repeated tightening and loosening test and the surface of a test sheet for a salt spray test. On the other hand, due to the absence of a dissolving medium, it had good biodegradability, and the biodegradability (BOD₂₈) of the entire lubricating coating composition in seawater was 36%, which greatly exceeded the minimum acceptable value of 20%, and was higher than that of the composition of Example 1 containing a dissolving medium.

In the repeated tightening and loosening test using a test threaded joint having a lubricating coating formed from the composition, 10 cycles of tightening and loosening which are required for a joint for tubing could be performed without the occurrence of galling both with a joint made of the carbon steel and one made of the 13Cr steel. In addition, in the salt spray test for 1000 hours using a test sheet having a lubricating coating formed from the composition, no rust was found on either the carbon steel or the 13Cr steel.

Comparative Example

A lubricating coating composition was prepared which contained 15% of mineral spirit as a dissolving medium, 72% (84.7 parts) of the same basic calcium sulfonate as used in Example 1, and 8% (9.4 parts) of petrolatum wax and 5% (5.9 parts) of a polyethylene resin powder (which is described in JP 2002-173692 A1 as a preferable lubricating additive due to its effect on improving galling resistance) both as additional lubricants. The parts in parentheses are the amounts of the respective components in part by mass based on 100 parts by mass of the total amount of the components in the composition excluding the dissolving medium.

This lubricating coating composition contained a dissolving medium and thus had a low viscosity and a high spreadability, so it was easy to apply and could be applied by brush coating to the surfaces of a test threaded joint to be used in a repeated tightening and loosening test and a test sheet for a salt spray test while remaining at room temperature. However, due to the presence of the dissolving medium which does not have good biodegradability, a high content of the basic lubricant, and the presence of a polyethylene resin powder, the biodegradability (BOD₂₈) of the entire lubricating coating composition in seawater was 12%, which was below the minimum acceptable value of 20%.

In the repeated tightening and loosening test using a test threaded joint having a lubricating coating formed from the composition, 10 cycles of tightening and loosening which are required for a joint for tubing could be performed without the occurrence of galling both with a joint of made of the carbon steel and one made of the 13Cr steel. In addition, in the salt spray test for 1000 hours using a test sheet having a lubricating coating formed from the composition, no rust was found on both the carbon steel and the 13Cr steel.

Thus, in this comparative example, the desired galling resistance and anticorrosive properties could be achieved since the composition which was used contained a large amount of a basic calcium sulfonate having good lubricity and anticorrosive properties along with polyethylene powder having good lubricity. However, the composition could not satisfy the desired biodegradability and therefore cannot be used in a country or region having strict environmental regulations.

Conventional Example

The performance of a compound grease as prescribed in API specifications BUL 5A2 which contained a large amount of heavy metal powder was evaluated as a conventional example. In the repeated tightening and loosening test using a test threaded joint having a lubricating coating formed from the grease, 10 cycles of tightening and loosening which are required for a joint for tubing could be performed without the occurrence of galling both with a joint made of the carbon steel and one made of the 13Cr steel. In addition, in the salt spray test for 1000 hours using a test sheet having a lubricating coating formed from the grease, no rust was found on either the carbon steel or the 13Cr steel.

Thus, it was confirmed that the galling resistance and anticorrosive properties of a lubricating coating composition according to the present invention which has good biodegradability are as good as those of a compound grease which contains a large amount of harmful heavy metals such as lead and hence cannot be used in a region having strict environmental regulations. 

1. A lubricating coating composition comprising at least one basic lubricant selected from a basic sulfonate salt, a basic salicylate salt, a basic phenate salt, and a basic carboxylate salt, the composition having a biodegradability value (BOD) of at least 20% when measured after 28 days in seawater.
 2. A lubricating coating composition as set forth in claim 1 which further comprises at least one additional lubricant selected from those having a higher biodegradability than that of the basic lubricant.
 3. A lubricating coating composition as set forth in claim 2 wherein the additional lubricant is selected from a fatty acid metal salt and a wax.
 4. A lubricating coating composition as set forth in claim 3 wherein the additional lubricant comprises at least one fatty acid metal salt and at least one wax.
 5. A lubricating coating composition as set forth in claim 4 which contains 0-30 mass % of a volatile organic dissolving medium, the remainder comprising, when its total amount is taken as 100 parts by mass, 55-75 parts by mass of the basic lubricant, 20-25 parts by mass of the fatty acid metal salt, and 10-20 parts by mass of the wax.
 6. A lubricating coating composition as set forth in claim 3 wherein the fatty acid metal salt is selected from alkaline earth metal salts of stearic acid or oleic acid.
 7. A tubular threaded joint constituted by a pin and a box each having a threaded portion and an unthreaded metal-to-metal contact portion as engaging portions, characterized in that the surfaces of the engaging portions of at least one of the pin and the box have a coating formed from a lubricating coating composition as set forth in claim 1 and having a thickness of at least 10 micrometers.
 8. A tubular threaded joint as set forth in claim 7 wherein the coating has a thickness of from 10 to 200 micrometers.
 9. A tubular threaded joint as set forth in claim 7 wherein the surfaces have a surface roughness of 5-40 micrometers Rmax.
 10. A tubular threaded joint as set forth in claim 9 wherein the surface roughness is formed by treating the surfaces by a method selected from sand or grit blasting, acid etching, phosphating, electroplating with iron or copper, and blast plating with zinc or a zinc alloy,
 11. A lubricating coating composition as set forth in claim 4 wherein the fatty acid metal salt is selected from alkaline earth metal salts of stearic acid or oleic acid.
 12. A lubricating coating composition as set forth in claim 5 wherein the fatty acid metal salt is selected from alkaline earth metal salts of stearic acid or oleic acid.
 13. A tubular threaded joint constituted by a pin and a box each having a threaded portion and an unthreaded metal-to-metal contact portion as engaging portions, characterized in that the surfaces of the engaging portions of at least one of the pin and the box have a coating formed from a lubricating coating composition as set forth in claim 2 and having a thickness of at least 10 micrometers.
 14. A tubular threaded joint constituted by a pin and a box each having a threaded portion and an unthreaded metal-to-metal contact portion as engaging portions, characterized in that the surfaces of the engaging portions of at least one of the pin and the box have a coating formed from a lubricating coating composition as set forth in claim 3 and having a thickness of at least 10 micrometers.
 15. A tubular threaded joint constituted by a pin and a box each having a threaded portion and an unthreaded metal-to-metal contact portion as engaging portions, characterized in that the surfaces of the engaging portions of at least one of the pin and the box have a coating formed from a lubricating coating composition as set forth in claim 4 and having a thickness of at least 10 micrometers.
 16. A tubular threaded joint constituted by a pin and a box each having a threaded portion and an unthreaded metal-to-metal contact portion as engaging portions, characterized in that the surfaces of the engaging portions of at least one of the pin and the box have a coating formed from a lubricating coating composition as set forth in claim 5 and having a thickness of at least 10 micrometers.
 17. A tubular threaded joint as set forth in claim 8 wherein the surfaces have a surface roughness of 5-40 micrometers Rmax. 